The invention relates to a plasma CVD process for coating, dome-like-shaped substrates or substrates approximating a dome shape, such as reflectors having an inside dielectric cold-light mirror coating. A dielectric and/or metallic coating system can be applied on the inside and/or outside face of such substrates. An apparatus for carrying out the process is also provided.
Reflectors are composed as a rule of arched glass substrates, generally dome-like in shape, having an inside reflection coating. The reflection coating may be composed of a metallic coating or, alternatively, if a specific spectral variation of the reflectivity is desired, of a dielectric coating system. Thus, it is possible to produce, for example, so-called cold-light mirrors, such as for example dental mirrors, which have a high reflectivity only in the visible spectral range but are transmissive for thermal radiation.
Dielectric coating systems having a selective spectral reflecting power are composed, in general, of coatings arranged alternatively one on top of the other and having a high and low reflective index. How such coating systems are to be constructed in detail, i.e., how many coating pairs have to be arranged one on top of the other and how the coating thicknesses are to be dimensioned to achieve a desired optical effect is known to the person skilled in the art and described, for example, in H. A. Macleod, Thin Film: Optical Filters, A. Hilger Ltd., London.
Normally, the dielectric coating systems are applied to the substrates by means of high-vacuum processes such as, for example, high-vacuum vapor deposition, cathode sputtering or electron-beam sputtering. In order to obtain uniform coating of the inside face of the substrate without complicated movement of a strongly arched substrate, a "gas scattering process" (K. Steinfelder et al., Vakuumtechnik 28 (1979), page 48) is as a rule used. In this, the vapor deposition is carried out under elevated pressure (about 10.sup.-3 mbar) of an additive gas which has the object of interrupting the linear movement of the vapor particles from the evaporation source towards the substrate by multiple collisions with the additive gas so that a preferred direction no longer exists in the movement of the vapor particles. So-called "soft" coating systems of ZnS/MgF.sub.2 coating pairs which are, however, sensitive to handling and so-called "semi-hard" coating systems of ZnS/SiO.sub.2 and ZnS/Al.sub.2 O.sub.3 coating pairs which, although resistant to handling, are not mechanically loadable, can be produced by this process.
In addition to the gas scattering process, a further high-vacuum vapor deposition process is known (H. K. Pulker, Coatings on Glass, Elsevier, Amsterdam 1984) in which the substrates perform a two-fold rotational movement on planetary mountings during the vapor deposition in order to achieve a uniform coating. Reflectors provided with the coating systems described above (diameter at the dome base approximately 5 cm) have, however, the disadvantage that they fail early at high atmospheric humidity and are not stable enough thermally still to withstand the high heat generated as rule by the halide incandescent lamps usually used, even at electrical powers of over 50 W.
Electron-beam evaporation in high vacuum makes it possible to produce SiO.sub.2 /TiO.sub.2 coating systems which meet high requirements in relation to their mechanical, chemical and thermal load-carrying capacity and are therefore normally described as so-called "hard" coating system. However, owing to the daily difficult production (for example, complicated substrate movement, substrate heating) these coating systems are several times more expensive on strongly arched substrates than the soft coatings.